1. Field of the Invention
The invention relates to a guide blade for steam turbines, having in the blade interior a cavity to which a vacuum is applied, and having at least one opening, which is provided with a porous cover having capillaries, for drawing off fluid from the trailing blade surface, preferably in the region of the blade edge, the opening leading into the cavity, and the cover being dimensioned in such a way that, during operation, all the capillaries are filled with the fluid to be drawn off.
2. Discussion of Background
In the low-pressure part of steam turbines, guide blades which are provided with a means for drawing off water are generally known. Guide blades of this type have openings, for example in the region of their trailing edge, which are formed as slits parallel to the blade edge or as bores and which lead into a cavity in the interior of the guide blade. The cavities of all the guide blades are connected to an annular duct, which is itself connected to the condenser of the steam-turbine plant. From the condenser, a comparatively low vacuum is applied to the cavity in the interior of the guide blade. By means of this vacuum, water condensed on the surface of the guide blade is drawn into the openings and passes on from there into the condenser. Without the water being drawn off in this way, water droplets would form on the guide blade and these would separate from the blade edge located downstream and come into contact with the moving blades of the steam turbine, which rotate at high speed. The moving blades can be eroded to a considerable extent by the water droplets coming into contact with them. This source of erosion can be eliminated by drawing off the water.
Through these openings, which have a comparatively large cross section, steam is also, as a rule, drawn off together with the water, a factor which brings about a reduction in the efficiency of the steam turbine. Furthermore, the edges of these openings disturb the steam flow along the guide blades.
In addition, it is known from DE-A1-2 038 047 to cover these openings, intended for drawing off the water, with a porous material which is permeable to liquid. By means of a comparatively large pressure gradient, the porous material is partly sucked empty in a continuous manner in order to create sufficiently open pores, into which the water wetting the blade surface is then drawn by means of the capillary action. The water is then drawn out of the pores into the blade cavity by means of the pressure gradient. When the comparatively large pressure gradient is utilized in this way, a certain proportion of the steam flowing around the blade is likewise drawn off as well despite the porous cover, a factor which results in a reduction in the output of the turbine. A comparatively large quantity of energy is consumed in order to maintain the comparatively large pressure gradient for drawing off the water.
DE-A1-195 04 631 discloses a guide blade of the type mentioned at the beginning. In this guide blade, a wall which withstands the application of a vacuum is formed by the porous cover and the liquid-filled capillaries. This wall is only permeable to the fluid where it is wetted by the fluid. The porous cover has a pore size which is matched to the fluid to be drawn off and to the vacuum. The openings for drawing off the fluid have a comparatively small effective cross section. During operation, there is always an impervious wall between the inner cavity to which a vacuum is applied, and the turbine interior, which is exposed to steam, which wall is composed of the porous material of the cover and the fluid contained in all the capillaries of this cover. This impervious wall constantly withstands the pressure gradient, designated as barrier pressure, between the turbine interior and the inner cavity to which a vacuum is applied. Although the water which wets the surface of the guide blade passes through this wall, steam cannot be entrained in the process, since, for the steam, the wall is solid and impermeable. The efficiency of the steam turbine is not significantly reduced by the water being drawn off at the guide blades, but its availability is substantially increased, since erosion phenomena caused by water droplets now no longer occur.
The openings for the covers are formed in the blade in the region of the trailing edge as a single-part or multi-part groove running parallel to the blade edge. The porous cover is connected to the guide blade by full-surface or spot brazing, by welding, by adhesive bonding and/or by mechanical peening. All these known fastening methods make it more difficult to exchange the cover if the need arises.